A polarizing plate has been usually used in a structure, in which a protective film is stacked on both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine. In this case, a triacetyl cellulose (TAC)-based film has been frequently used as the protective film because the film is excellent in optical transparency or moisture permeability.
Meanwhile, as a liquid crystal display device has recently been developed into a mobile device such as a laptop personal computer, a cellular phone and a car navigation system, it is required that a polarizing plate which constitutes a liquid crystal display device is thin and lightweight. However, in a polarizing plate in which a TAC film and the like are laminated as the protective film as described above, it is difficult to maintain a thickness of the protective film at 20 μm or less from the viewpoint of handling property or durability performance during the operation, so that there is a limitation for the liquid crystal device to be thin and lightweight.
In order to solve the aforementioned problems, a technology has been suggested, in which a transparent thin film layer is formed by forming a protective film on only one surface of a polarizer, and coating an active energy ray curable composition on the other surface thereof. Meanwhile, most of the transparent thin film layers, which have been suggested so far, secure adhesive strength through a hydrogen bond between a hydroxy group of a polarizer formed of a PVA-based resin and a hydrophilic functional group of a transparent thin film layer. However, since water molecules may also form a hydrogen bond with a hydroxy group of the polarizer formed of a PVA-based resin or a hydrophilic functional group of a transparent thin film layer in a high moisture atmosphere when a hydrogen bond is used as described above, there is a disadvantage in that the frequency of hydrogen bonds between a hydroxy group of a polarizer formed of a PVA-based resin and a hydrophilic functional group of a transparent thin film layer is decreased, and accordingly, adhesive strength deteriorates.
In order to solve the aforementioned problems, studies to secure adhesive strength have been actively conducted through a covalent bond instead of a hydrogen bond, and among them, particularly, studies on an active energy ray curable composition including an epoxy compound have been most actively conducted. In the case of an active energy ray curable composition including an epoxy compound, a ring-opening reaction between a hydroxy group of a polarizer formed of a PVA-based resin and an epoxy compound occurs during the curing process by irradiation of UV rays, a covalent bond is generated by the ring-opening reaction, and accordingly, excellent adhesive strength may be secured even in a high moisture atmosphere. However, since an active energy ray curable composition including the epoxy compound is cured by a cationic method instead of by a radical method, the curing rate is slow, and there also many disadvantages in preparation process because the degree of curing is also low.
Therefore, there is a need for a new polarizing plate which may secure adhesive strength through a covalent bond instead of a hydrogen bond, and thus has excellent adhesion between a polarizer and a protective layer, is excellent in water resistance and heat resistance, and may be manufactured as a thin type.